The marketing material for SA400 shows a digital feedback but I don't see that in the ST specs. I am not clear if this is an addition by SMSL or it is part of ST amplifier chip that they don't talk about.
This is a trifle mysterious. The STa516BE isn't a complete amplifier. It is just the power output stage. It requires a modulator to create the PWM control. ST make a couple of fully integrated modulators, the obvious one being the STA321. This device has both analog and digital imputs and provides the full monty of tone controls and volume control, all in the digital domain. But it is clear that the SMSL 400 doesn't use digital volume or tone controls. They proudly use the JRC NJW1119A tone control and NJW1195 volume control, which are analog domain controls.
This suggests that SMSL have implemented a different modulator to the standard ST parts.
In the pic below from the marketing material the STa516BE is the box labelled
Power Stage. The rest is SMSL. They are very coy about what is going on here. It is a hybrid negative feedback loop. There is much black magic here. Use of real true negative feedback is challenging to say the least. Stability is going to be a slippery beast. Hypex have
patent *
on how to do it that derives from their research on self oscillating PWM amplifers (basically transforming their analog topology into a digital dual.) How SMSL does it would be very interesting to know.
The diagram however shows a digital input to the system. That isn't consistent with the use of the analog volume and tone control chips. So one suspects there is a lot more that the marketing doesn't get quite right.
A pic of the internals would be most welcome.
*Editing to correct this. It isn’t a patent but a paper. The paper is cited in the patent Axign have.
- Mouton, Toit , et al., “Digital Control of a PWM Switching Amplifier with Global Feedback”, AES 37th International Conference, 2009, 1-10.
et al is Bruno Putzeys.